Transmission of information by the use of light over optical fibers is widely used in long-haul telecommunication systems. Optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Fibers are substituted for other transmission media and all signal processing is done electronically, resulting in lowered cost and high quality digital transmission.
As fiber optic applications technology develops direct optical processing of signals without conversion to electronic signals will be required. Optical fiber systems will be applied in computer networks, for example, in multiple access computer networks. Such applications will require optical fiber devices such as amplifiers, multiplex/demultiplexers, splitters, couplers, filters, equalizers, switches and other optical signal processors.
An economical low-loss, easily and reproducibly manufactured single-mode optical fiber filter, the design of which can be adapted to a desired bandwidth, FSR and finesse is an important component for such fiber optic systems. A fiber Fabry-Perot (FFP) interferometric filter is such a component.
The Fabry-Perot (FP) Interferometer was first described by C. Fabry and A. Perot in 1897 (Ann. Chem. Phys., 12:459-501) and has since found wide use in a variety of applications of optical filters. The basic structure and operation of the FP interferometer is well-known in the art and is described in many physics and optics texts (see, for example, E. Hecht "Optics" 2nd. Edition (1987) Addison-Wesley, Reading Mass., p. 369). This interferometer consists of an optical cavity formed between two typically highly reflecting, low-loss, partially transmitting mirrors. Lenses are typically used to collimate divergent optical beams for processing through the FP interferometer.
While single-mode optical fibers can be used with lensed conventional FP interferometers, lenses with large beam expansion ratios are required and result in reduced stability and poor optical performance. The adaptation of FP cavities for optical fiber filters has been hindered by the lack of practical designs for FFPs with appropriate optical properties. Recently, FFPs which possess optical properties suitable for telecommunication applications have been described. These FFPs consist of two highly reflective, preferably plane-parallel mirrors, forming the optical cavity through which, in most cases, a length of single-mode optical fiber extends. This basic design eliminates the need for collimating and focusing lenses, improves stability and optical performance and makes the FFPs compatible with single-mode optical fibers and other fiber devices.
In 1987, J. Stone and L. W. Stulz described three configurations of FFP interferometric filters (Elect. Lett., 23(15):781-783, 1987) that span a wide spectrum of bandwidths and tuning ranges. The Type I FFP is a long cavity FFP in which mirrors are deposited at the ends of a continuous fiber. The minimal cavity length is about 1 cm (FSR of about 10 GHz), so that this long cavity device is not necessarily important for telecommunication applications. In the Type I FFP, the fiber can be stretched by piezoelectric transducers (PZTs) to produce tuning of the bandwidth (BW) over the free spectral range (FSR).
The Type II FFP of Stone and Stulz is a gap resonator which has no fiber inside the optical cavity and so can exhibit significant losses. Due to such losses, the useful cavity length of this type of FFP is less than about 5 .mu.m. The Type II FFP is also not well-suited for telecommunication applications.
The Type III FFP is better suited to telecommunication applications than the Type I and II FFPs. This type of FFP has an internal waveguide interposed between external fiber ends. Mirrors are positioned at an external fiber end and at one end of the waveguide. The waveguide is comprised within the optical cavity. The optical cavity also contains a gap, the width of which is fixed or can be changed to tune the filter.
The ferrule components and waveguide of Type II and III FFPs must be axially aligned to high precision in order to minimize transmission loss. Type II and III FFPs are the subject of U.S. Pat. No. 4,861,136. This patent relates to FFPs which are tuned by use of PZTs to change the cavity length. In order to use PZTs to change cavity length without detriment to alignment elaborate alignment brackets and fixtures were necessary.
Clayton and Miller, U.S. Pat. No. 5,062,684, describe fiber ferrule assemblies which are useful in the construction of FFPs. The ferrule assemblies described are composed of two wafered ferrules, each of which contains an embedded mirror. Each wafered ferrule contains an axial passageway, along its longitudinal axis, in which an optic fiber is positioned. A mirror is deposited at the ferrule end face to which the wafer is bonded. The wafered ferrules are aligned in an alignment fixture to maximize transmission through the fibers and such that an optical cavity is formed between the two mirrors. The dual wafer ferrule assembly is said to protect mirrors from inadvertent damage. Fixed and cavity length tunable FFP having a dual wafered ferrule assembly are described.
While dual-wafered ferrule assemblies meet certain of the requirements for practical FFP designs with useful optical properties, the dual-wafered ferrule FFP is limited to a maximum FSR of about 8,000 to 10,000 GHz. Further, the optical cavity of a dual-wafered ferrule FFP contains two glass/air interfaces at the wafer end faces which can have a significant effect on the variation of FSR with wavelength. This can also lead to significant variations in finesse and BW among ferrule assemblies in which the reflectivities of the mirrors are the same. The ability to efficiently reproduce desired optical properties in the manufacture of FFPs can be significantly impaired by this variability. The difficulty in controlling the finesse of a dual-wafered ferrule FFP increases markedly in high finesse filters, i.e., in filters designed to have a finesse of 100 or more. Sensitivity to alignment offset is perhaps the most important optical property of an FFP filter. Since alignment error is always present, and the level of sensitivity to this error determines the corresponding insertion loss, ruggedness and long-term stability of the device. Increasing the uniformity of manufacture of FFP ferrule elements and increasing the ability to control the optical properties of these elements results in significantly increased production yield of FFPs having desired specifications and, thus, leads to significantly decreased manufacturing costs.